Materials, both organic and metallic, capable of being rendered heat recoverable are well known. An article made from such materials can be deformed from an original, heat-stable configuration to a second, heat-unstable configuration. The article is said to be heat recoverable for the reason that, upon the application of heat, it can be caused to revert from its heat-unstable configuration to its original, heat-stable configuration.
Among metals, for example certain alloys of titanium and nickel, the ability to be rendered heat recoverable is a result of the fact that the metal undergoes a reversible transformation from an austenitic state to a martensitic state with changes in temperature. An article made from such a metal, for example a hollow sleeve, is easily deformed from its original configuration to a new configuration when cooled below the temperature at which the metal is transformed from the austenitic state to the martensitic state. This temperature, or temperature range, is usually referred to as the M.sub.s temperature. When an article thus deformed is warmed to the temperature at which the metal reverts back to austenite, referred to as the A.sub.s temperature or range, the deformed object will revert to its original configuration. Thus, when the hollow sleeve referred to above is cooled to a temperature at which the metal becomes martensitic, it can be easily expanded to a larger diameter, for example, by using a mandrel. If the expanded sleeve is subsequently allowed to warm to the temperature at which the metal reverts back to its austenitic state, the sleeve will revert to its original dimensions.
Ordinarily, such a sleeve would recover all or substantially all of the deformation, i.e., it would revert completely to its original dimensions. However, it should be noted that under certain circumstances the article might be deformed to such an extent that all of the deformation cannot be recovered on heating. Alternatively, if something, e.g., an intervening rigid substrate having a greater external dimension than the internal pre-deformation dimensions of the sleeve is interposed within the sleeve, the sleeve cannot recover to its original dimensions. Any dimensional change up to the maximum available which an article can recover absent any intervening substrate is called the heat recoverable strain. That portion of the heat recoverable strain which an intervening substrate or other agency precludes recovery of, is referred to as unresolved recovery. Finally, any deformation which exceeds the maximum available heat recoverable strain is said to effect non-recoverable strain.
That the titanium nickel alloys referred to above possess the property of heat recoverability has been known for many years. More recently, Brook et al, for example in U.S. Pat. No. 3,783,037, the disclosure of which is incorporated by reference, have disclosed a method for producing a heat recoverable article in which an alloy comprising an inter-metallic compound that undergoes a diffusionless transformation into a banded martensite upon cooling with or without working is deformed after appropriate heat treatment. On reheating the article, it at least partly resumes its original shape. The alloys preferred by Brook et al are copper based alloys which transform into a martensite of pseudo-cubic symmetry. The preferred alloys include the binary copper-zinc and copper-aluminum systems and the ternary copper-aluminum-zinc, copper-aluminum-tin, copper-zinc-silicon, copper-aluminum-manganese, copper-aluminum-iron and copper-aluminum-nickel systems.
In U.S. Pat. No. 3,783,037 (Col. 8, In. 63 et seq.) Brook et al note in respect to the copper-aluminum-zinc system that "... as there is progressive increase in the aluminum content and decrease in the zinc content..., the maximum ductility that can be produced in the ternary alloys when deformed at or near the M.sub.s decreases." They further note that as the aluminum level increases, the maximum obtainable heat recoverable strain decreases. For example, in alloys of the compositions (by weight) 72% copper, 22% zinc and 6% aluminum and 75.7% copper, 17% zinc and 7.5% aluminum, the maximum heat recoverable strain was reported to be 4.8% and 4.0%, respectively.
The clear teaching of this patent is therefore that the aluminum content of the alloy should be reduced as much as possible to achieve enhanced heat recoverable strain. Unfortunately, I have found that, unknown to the prior art, reducing the aluminum content has a severe adverse effect on the stability of the article under conditions of unresolved recovery. Additionally, if one follows the teaching of the prior art and avoids ternary alloys containing significant quantities of aluminum, limitations are encountered in hot working. In particular, low energy input hot working requires avoidance of a second phase in the structure. Unfortunately, low aluminum content alloys must be maintained at very high temperatures, e.g. at least in excess of 650.degree. C., to be in the one-phase beta condition. At such high temperatures, tool life is shortened and the avoidance of coarse grain size in the product is very difficult
If a heat recoverable article is recovered onto a substrate such that the substrate prevents full recovery of the article to its original configuration, i.e., under conditions of unresolved recovery, then the residual strain results in a stress in the article. I have now discovered that all copper alloy compositions having the .beta.-brass structure are more or less unstable if complete recovery is prevented. Thus, I find that at moderate temperatures such as would typically be seen during service, for example, in hydraulic or electrical applications in aircraft, the residual stress in incompletely recovered articles will decay steadily to zero such that after a certain period of time the recovered object, for example, a sleeve recovered about a substrate, can be easily removed from the substrate.
Inasmuch as heat recoverable metals find their greatest utility in applications where they exert a high degree of compressive or other form of stress, it will be readily recognized by those skilled in the art that the stress relaxation process described above is a considerable impedement to the wide spread use of these metals. For example, parts made from the binary alloys and the specific ternary alloys described in the Brook et al patent mentioned above, when prevented from recovering completely to an initial configuration under conditions of about 4.0% unresolved recovery, exhibit complete stress relaxation at 125.degree. C., in less than 1,000 hours (equivalent to within 100 hours at 150.degree. C.) so that they are essentially useless in many applications.
In the aforementioned patent, Brook et al also describe a process they term "reversible heat recoverable strain" in a copper-zinc-tin alloy which had an M.sub.s of -70.degree. C. A sample of this alloy was quenched from 800.degree. C., deformed below its M.sub.s and allowed to recover by heating above its A.sub.s. It was noted that there was partial recovery of the strain that had been induced in the alloy by its deformation as it was heated into the range in which the alloy reverted to its austenitic state. On further heating to 250.degree. C., the specimen surprisingly changed shape by immediately moving back toward the deformed configuration. This alloy was considered by them to be unique in this regard. I have found that this phenomenon of reverse recovery is by no means unique to the particular alloy reported but is, in fact, prevalent among many of the reported prior art compositions. Such phenomenon is merely a particularly severe manifestation of the unresolved recovery induced instability (stress relaxation) hereinafter discussed in greater detail, i.e. a loss of stress even under zero restraining force. Needless to say, none of my instantly claimed alloys manifest such a phenomenon.
Therefore, although a wide variety of .beta.-brass type copper alloy compositions capable of being rendered heat recoverable are known to the prior art, those compositions possess serious shortcomings severely limiting their use.
Accordingly, one object of this invention is to provide improved .beta.-brass type alloys.
Another object of this invention is to provide heat recoverable articles of .beta.-brass type alloys that will exhibit long term stress stability when recovered under conditions so that a level of unresolved recovery remains.
Yet another object of this invention is to provide heat recoverable articles of .beta.-brass type alloys that will maintain a stress for greater than 1,000 hours at 125.degree. C. or for greater than 100 hours at 150.degree. C.